Welded heat exchanger with grommet construction

ABSTRACT

A heat exchanger comprising a core assembly having a plurality of tubes connected by an array of fins, with ends of the tubes extending from the fin array on at least one side of the core assembly and resilient, polymeric grommets disposed around, the tube ends. The heat exchanger includes a tank portion for receiving fluid entering or leaving the core assembly and a header portion having openings to receive the tube ends and grommets to create a sealed, fluid-tight tube-to-header joint. There is provided a reinforcing member across the opening of the tube ends extending from the tube end opening to the portion of the tube end around which the grommet is disposed without substantially extending into the fin array portion of the core assembly. The header portion comprises a substantially planar base having openings therein with side and end walls extending out of the plane of the base and connected by one or more fluid-tight welds to the tank portion. The side and end walls extend a distance sufficient to dissipate heat generated in making the welds and keep the heat generated from adversely affecting sealing ability of the grommets when the welds are made between the tank and header portion with the tube ends and grommets received in the header portion openings.

This application is a division of U.S. application Ser. No. 09/266,206,filed 03/10/99.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to heat exchangers and, more particularly, tomotor vehicle heat exchangers utilizing grommets in the tube-to-headerjoints and tanks welded to the header.

2. Description of Related Art

Heat exchangers, particularly those utilized in motor vehicles, may beliquid-to-air heat exchangers, (e.g., radiators for engine coolant, airconditioning condensers and evaporators, and oil coolers) or may beair-to-air heat exchangers (e.g charge air coolers). Liquid-to-air andair-to-air heat exchangers are typically composed of an inlet tank ormanifold, an outlet tank or manifold, and a large number of tubesextending between the tanks or manifolds which carry the fluid to becooled. Headers are normally provided on the tubes for mechanicalattachment and fluid connection to the tanks. Fins attached to the tubestransfer heat between the liquid or gas inside the tubes and the ambientatmosphere outside. A mechanical framework or structure is usuallyincluded to provide structural strength to the assembly and to providemeans for mounting the unit to the vehicle or other machinery on whichit is used.

The tubes utilized may be either round or oval, or may be oval withcircular ends. Prior art methods of welding tube-to-header joints aredisclosed, for example, in U.S. Pat. No. 5,407,004, the disclosure ofwhich is hereby incorporated by reference.

In use, heat from the hot liquid or air within generally causes thetubes to expand and grow in length due to thermal expansion. Since thetanks or manifolds are fixed with respect to each other by the unitframework or structure, the growth in length of tubes places highmechanical stresses on the tanks and the associated headers,particularly in the area of the joints between the tubes and headers. Inaddition, the pressure of the hot liquid or hot air within the heatexchanger tends to distort the tanks or manifolds and headers, creatingfurther stresses on the tube-to-header joints. The combination of stressresulting from thermal expansion and internal pressure can result inearly failure of heat exchangers. Cracks in the joints between the tubesand the headers or in the tubes immediately adjacent to these joints arethe most common mode of failure. Many approaches have been taken toavoid heat exchanger failures due to thermal expansion and internalpressure. Most approaches fall into one of two categories: 1) thosewhich improve the strength of the areas prone to failure and 2) thosewhich provide resilience in the areas prone to failure. Approaches whichprovide resilience have appealed to designers because, they provide asolution to the stresses of thermal expansion and internal pressureswith a greater economy than any approach which must provide morematerial to, achieve an improvement in strength.

Engine cooling radiators for vehicles have sometimes been designed withresilient tube-to-header joints. Locomotive radiators have beenmanufactured by the assignee of the present invention for over thirty(30) years using headers of special resilient design. In this design,metallic headers have oversized holes or openings in them to receiveoval brass tubes extending from the radiator core. Within the openingsin the header there are placed oval brass ferrules. These ferrules arebonded to the header by molded silicone rubber. The ferrules are thensoldered to the core tubes extending there through to form a leak-free,resilient joint between the tubes and the headers. Fins of the flatplate-type design have collars fitted around the tubes. The headers aremechanically attached to tanks, such as by bolting. While this has beenan extremely effective design under typical operating conditions forlocomotives, it is expensive to produce.

In the 1970's, radiators for automobiles were produced which utilizedround aluminum tubes, aluminum plate fins, aluminum headers and plastictanks. A sheet of molded rubber provided resilient grommets at each tubehole in the header, and also provided a gasket for sealing the headersto the plastic tanks, which were attached to the headers by means ofcrimped tabs on the headers. The insertion of the tubes into the rubbergrommets in the header holes compressed the rubber of the grommetsproviding a resilient sealing attachment of the tubes to the headers.However, considerable force was required to insert all the core tubesinto the header holes simultaneously. This design was limited torelatively small units because of the problems of core and headerdistortion during assembly and because of the close tolerances whichwere required to accomplish the mating of the core tubes to the headerwith the desired amount of grommet compression.

Other radiators have also utilized rubber grommets in theirtube-to-header joints. These radiators have been designed aroundindividual finned tubes having round ends and oval cross-sections whichare finned along most of their length. As in the previous design,sealing of the tubes to the header was accomplished by compression ofthe grommets between the tubes and the header. However, in thisalternative design, the tubes were assembled to the headers individuallythereby avoiding high assembly forces. This allowed the construction ofvery large radiators for heavy construction equipment. However, it hasbeen found that the use of tubes with round ends limits this design tocores having rather wide tube spacing, which results in relatively poorthermal performance compared to most radiator core designs.

U.S. Pat. Nos. 4,756,361 and 5,205,354 describe a radiator whichutilizes circular tubes and tube ends which extend through siliconerubber grommets which are disposed in openings within a header plate.U.S. Pat. Nos. 5,052,475 and 5,226,235 disclose use of circular grommetsto seal circular tubes into soldered tanks and welded tanks,respectively. British Patent No. 29,777 discloses the use of round tubesand grommets with a tube plate cast integrally with the header.

Currently, air-to-air heat exchangers using brazed aluminum cores havingoval tubes are being produced commercially. Aluminum headers havingoversized oval openings are welded to cast aluminum manifolds. Ovalsilicone rubber grommets, otherwise similar to those described in theaforementioned '361 patent, are inserted into the openings in theheaders of the welded tanks.

In the aforementioned '361 patent, the headers are intended to beconnected to inlet and outlet tanks by means of bolting with a sealinggasket, or other similar means. U.S. Pat. No. 5,226,235 describes aradiator made up of a unitary core consisting of tubes and fins,connected to tank assemblies which consist of welded tank and headerassemblies into which resilient grommets have been inserted afterwelding. This '235 patent provides advantages over the '361 patent inthat the welded tank and header assembly is more compact than oneassembled by bolting with a sealing gasket. In addition, a welded tankdesign is quite flexible and can be quickly made up to suit anyapplication. However, a difficulty with the '235 patent is that itrequires that all welding of the tank/header assembly is to be completedbefore assembly of the resilient grommets to that assembly. Thins isstated to be necessary to prevent heat damage to the resilient grommets.However, it makes assembly of the unitary core assembly to thetank/header assemblies particularly difficult, since one can only seeone side of the grommets and access to the other side of the grommets toaid in assembly is denied. For instance, it would aid insertion of thecore tubes into the grommets if the tube ends were fitted with removablebullet noses. However, in the one-piece tank/header design, retrieval ofthe bullets after insertion is practically impossible.

Additionally, access to the side of the grommets remote from the tubeinsertion side can be helpful to tube insertion in other ways. Forexample, supporting or backing-up the remote side of the grommet duringtube insertion makes tube insertion easier and prevents pushing out ofthe grommets. This is, of course, impossible with the one piecetank/header assembly described in the '235 patent.

It would be advantageous to have the critical tube-to-header joints madeand inspected with full access to both the airside and fluid side of thejoints. The core assembly can bet separately tested and corrections canbe made to the tube-to-header joints, if necessary, before the tanks areattached.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide an improved heatexchanger and method of making such heat exchanger which utilizesgrommets in tube-to-header joints, which header may be welded to thetank after insertion of the grommets.

It is a further object of the present invention to provide an improvedmethod of assembly of a heat exchanger in which the core assembly can beseparately tested and corrections can be made to the tube-to-headerjoints, if necessary, before the tanks are attached.

SUMMARY OF THE INVENTION

The above and other objects and advantages, which will be apparent toone of skill in the art, are achieved in the present invention which isdirected to, in a first aspect, a heat exchanger comprising a coreassembly having a plurality of tubes connected by an array of fins, withends of the tubes extending from the fin array on at least one side ofthe core assembly, and resilient, polymeric grommets disposed around thetube ends. Preferably, the grommets of the heat exchanger are made of aheat-resistant silicone rubber.

The heat exchanger includes a tank portion for receiving fluid enteringor leaving the core assembly and a header portion having openingsreceiving the tube ends and grommets to create a sealed, fluid-tighttube-to-header joint. The header portion comprises a base havingopenings therein and side walls extending out of a plane of the base andconnected by one or more fluid-tight welds to the tank portion. The sidewalls extend a distance sufficient to dissipate heat generated in makingthe welds and keep the heat generated from adversely affecting thesealing ability of the grommets when the welds are made between the tankand the header portion with the tube ends and grommets received in theheader portion openings.

The header portion of the heat exchanger may be comprised of a unitarysheet of metal having folded or drawn side walls extending from thebase. The side and end walls of the header portion may be joined atcorners thereof to create an open box structure. Preferably, the headerbase is substantially planar while the side and end walls aresubstantially perpendicular to the plane of the base.

In the preferred embodiment, the structure also includes a reinforcingmember across the opening of the tube ends extending from the tube endopening to the portion of the tube end around which the grommet isdisposed without substantially extending into the fin array portion ofthe core assembly.

In another aspect, the present invention relates to a heat exchangercomprising a core assembly having a plurality of tubes connected by anarray of fins, with ends of the tubes extending from the fin array on atleast one side of the, core assembly and resilient, polymeric grommetsare disposed around the tube ends. The heath exchanger includes a tankportion for receiving fluid entering or leaving the core assembly and aheader portion having openings to receive the tube ends and grommets tocreate a sealed, fluid-tight tube-to-header joint. The header portioncomprises a substantially planar base having openings therein with sideand end walls extending out of the plane of the base and connected byone or more fluid-tight welds to the tank portion. The side and endwalls extend a distance sufficient to dissipate heat generated in makingthe welds and keep the heat generated from adversely affecting sealingability of the grommets when the welds are made between the tank andheader portion with the tube ends and grommets received in the headerportion openings.

In yet another aspect, the present invention provides a method of makinga heat exchanger. The method comprises initially providing a coreassembly having a plurality of tubes connected by an array of fins withthe ends of the tubes extending from the fin array on at least one sideof the core assembly and resilient, polymeric grommets disposed aroundthe tube ends. The grommets provided may be comprised of aheat-resistant silicone rubber. Also provided are a tank, having an openend portion for receiving fluid entering or leaving the core assembly,and a header portion with a base having openings for receiving the tubeends and side walls extending out of a plane of the base. In thepreferred embodiment, the side walls are extended a distance sufficientto dissipate heat generated in welding the tank to the header portionand keep the heat from adversely affecting the sealing ability of thegrommets when the welding is performed with the tube ends and grommetsreceived in the header portion openings.

The method also includes attaching the header portion to the coreassembly by receiving the tube ends and grommets in the header portionopenings to create a sealed, fluid-tight tube-to-header joint. Theheader portion is then welded to the tank open end portion to create oneor more fluid-tight welds. In the preferred embodiment, the weldingtakes place with the tube ends and grommets received in the headerportion openings without adversely affecting the sealing ability of thegrommets in the,tube-to-header joint.

The method may further include the step of either folding edge portionsor drawing portions of the unitary sheet of metal comprising the headerportion to create the side walls extending from the base, and may alsoinclude joining the side walls and end walls at corners thereof tocreate an open box structure. The side and end walls may be folded topositions substantially perpendicular to the plane of the substantiallyplanar base.

In the preferred embodiment, the tube ends include a reinforcing memberinserted across the opening of the tube extending from the tube endopening to the portion of the tube end around which the grommet isdisposed without substantially extending into the fin array portion ofthe core assembly.

A further aspect of the invention relates to a heat exchanger comprisinga core assembly having a plurality of tubes connected by an array offins, with ends of the tubes extending from the fin array on at leastone side of said core assembly and resilient, polymeric grommetsdisposed around the tube ends. A reinforcing member is disposed acrossthe opening of at least a portion of the tube ends and extends from thetube end opening to the portion of the tube end around which the grommetis disposed without substantially extending into the fin array portionof the core assembly. The heat exchanger includes a tank for receivingfluid entering or leaving the core assembly and a header portion havingopenings receiving the tube ends and grommets to create a sealed,fluid-tight tube-to-header joint.

The header portion of this embodiment comprises a base having openingsherein and side walls extending out of a plane of the base and connectedby one or more fluid-tight welds to the tank. The side walls extend adistance sufficient to dissipate heat generated in making the welds, andkeep the heat from adversely affecting sealing ability of the grommetswhen the welds are made between the tank and header portion with thetube ends and grommets received in the header portion openings.

BRIEF DESCRIPTION OF THE DRAWING

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a perspective view of the preferred unitary brazed core blockemployed in the present invention.

FIG. 2 is a perspective view of the preferred box header employed in thepresent invention.

FIG. 3 is a perspective view of the preferred stamped blank of the boxheader of FIG. 2.

FIG. 4 is a perspective view of the preferred molded grommets employedin the present invention.

FIG. 5 is side elevational view of one step in assembling the tubes ofthe core assembly into the openings of the header plate to make the heatexchanger of the present invention.

FIG. 6 is side elevational view of an assembly step subsequent to thatof FIG. 5.

FIG. 7 is side elevational view of an assembly step subsequent to thatof FIG. 6.

FIG. 8 is a perspective view of the box header of FIG. 2 assembled ontothe unitary brazed core block of FIG. 1.

FIG. 9 is a top plan view of the tubes fitted into the box header ofFIG. 8.

FIG. 10 is an end elevational view of a tube fitted into the box headeralong line 10—10 of FIG. 9.

FIG. 11 is a side elevational view of a tube fitted into the box headeralong line 11—11 of FIG. 9.

FIG. 12 is an exploded view of a preferred tank to be welded onto thebox header of FIG. 2.

FIG. 13 is a perspective view of the tanks assembled onto the boxheaders and unitary brazed core block assembly of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-13 of the drawings in whichlike numerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

This invention is directed primarily to air cooled radiators whichutilize a liquid coolant for automotive applications. However, it mayalso be utilized for any other type of heat exchanger, for example,those discussed in the background of the invention such as oil coolers,charge air coolers and the like. The present invention provides astructural assembly and method which permits the header to be welded tothe tank after insertion of the grommets into the header openings,without heat damage to the grommet material. Preferably oval tubes areutilized in the heat exchanger for close tube spacing for optimum heattransfer performance of the core, although other tube shapes andcross-sections may be utilized.

As shown in FIG. 1, the heat exchanger of the present inventionpreferably utilizes a core block or assembly 20 which comprises an arrayof parallel tubes 22, preferably oval tubes, between which areinterposed an array of heat transfer fins 24. These fins May be of theflat type or serpentine type as shown in the core depicted in FIG. 1.Any of these fin styles may include louvers to enhance heat transfer.The tubes and fins are typically formed of aluminum, brass, copper orother thermally conductive metal or alloy. Serpentine fins 24 may extendin a serpentine pattern wherein the strips are configured with aplurality of alternating bends between, adjacent tubes 22. The root ofthe bend is generally secured by brazing or soldering to the tube.Alternatively, the fins may be made of sheet metal which has a collarformed about a hole. The tubes may be inserted through the collaropening and a plurality of fins may be stacked in order to make up thefin array within the core. The ends 23 of tubes 22 extend beyond the finarray of core to connect to the headers and tanks utilized in the heatexchanger.

The preferred header employed in the present invention is depicted inFIG. 2. Header 30 comprises a generally rectangular body having asubstantially flat or planar base portion 38 of length L and width Wwithin which are formed header openings 36 which conform generally tothe configuration of tube ends 23. The header portion openings 36 arepreferably oval and arranged in a single row as shown, although multiplerows of tubes may be employed. As used herein, the term “oval” refers toany non-circular shaped axial cross-section (i.e. perpendicular to theaxis of the tube) having a generally smoothly curving periphery such asan ellipse or a rectangle with rounded corners, or other obround or eggshape. Being of oval cross-sectional shape, such tubes and openings willhave a diameter in one direction greater than the diameter in another(usually perpendicular) direction, which are referred to herein as the“major diameter” and “minor diameter” respectively. While oval shapedtubes and, consequently, oval shaped header openings and grommets arepreferred, other tube opening and grommet configurations can be used,such as circular or rectangular. For reasons discussed further below, inorder to accommodate the grommets, the header openings 36 are largerthan the tube ends 23. The width W of header plate portion 38 is onlyslightly greater than the major diameter of header openings 36 tominimize header flexing and achieve maximum header portion strength.

Forming a box-type configuration to the header 30 are upstandingvertical side walls 32 and end walls 34 which extend upward from theplane of base 38. Side walls 32 face each other along opposite edgesalong the long sides of plate portion 38, corresponding to length L,while vertical upstanding end walls 34 face each other along the shorterends of plate portion 38, corresponding to width W. Side walls 32 havefree edges 32 b at the upper portions thereof while end walls 34 havefree edges 34 b along the upper portions thereof. Edges 32 b and 34 b ofheader 30 are connected by welding to the tank portions of the heatexchanger as will be discussed further below. In order to permit properheat dissipation and to prevent excessive heat buildup in the vicinityof header openings 36, it is preferred that the height H of side walls32 and end walls 34 b are carefully controlled. For an example of aheader utilized in motor vehicles, it is preferred that the height ofthe side and end walls be at least about one-half (½) inch (1.25 mm),more preferably at least about one (1) inch (2.5 mm). The header sideand end wall height is also preferred to be less than about two (2)inches (5.0 mm), more preferably less than about one and one-half (1½)inches (3.75 mm). The header side and end wall height should be highenough to avoid excessive temperatures to develop in the grommets as aresult of welding at edges 32 b, 34 b to the tank, but not so high as torequire excessive material. The header may be formed by any conventionalmanufacturing process, such as casting, stamping or drawing.

The preferred stamped blank of header 30 is depicted in FIG. 3. Theheader plate portion 38, side walls 32 and end walls 34 are preferablyformed from a solid unitary sheet of metal, for example copper, brass oraluminum. Vertical side walls 32 are formed by bending or folding alongthe line indicated by 33 while vertical end walls 34 are formed bybending or folding along the lines indicated by 35. Side wall 32 endportions 32 a then contact end wall 34 side portions 34 a along avertical seam which may be joined and sealed by welding to form corners.Such welding is preferably performed prior to insertion of the grommetssince the corners formed by end portions 32 a, 34 a are relatively closeto the header openings 36.

For drawn headers, the side walls 32 and end walls 34 (FIG. 2) are drawnfrom a unitary sheet of metal and extend upwards from the base or plateportion 38.

Grommets are the preferred sealing means between the tubes and theheader portion openings. The preferred grommet of the preferredinvention is depicted in FIG. 4. Grommet 40 is preferably made of aheat-resistant silicone rubber or other resilient, flexible or compliantpolymer or other material which provides good sealing against the othercomponents of the heat exchanger, and which has suitable heat resistanceto avoid affecting the grommet's sealing ability when the header iswelded to the tank while the grommets are in the header openings. Thepreferred grommet is made from silicone rubber, such as compound ST125785-V-RED, which may be obtained from Dow Corning Corporation. Thesilicone rubber is a high-temperature silicone rubber which includes avulcanizing agent and meets ASTM D2000. The other pertinent propertiesof the preferred grommet material were as shown in Table 1 below.

TABLE 1 Property Value Hardness Shore A 50 Tensile Strength, kgf/cm² 90Tear Strength, kgf/cm 18 Elongation, % 300 Specific Gravity 1.16Shrinkage Rate, % 3.3 Compression Set, %, 22hr. @177° C. 11 TemperatureRange, ° C. minus 55 to plus 250 Change After Heat Aging, 250° C./72hr.:Hardnesss, Points 2 Tensile, % −19 Elongation, % −10

In laboratory tests with headers of the invention, it was determinedthat the grommets were able to withstand temperatures up to 500° F.-600°F. (260° C.-315° C.). Grommets 40 comprise a body portion having anopening therein to receive the tube from a heat exchanger core. Aradially outward extending lip or flange is disposed at a first upperend of the body portion and a radially inwardly extending lip or flangeis disposed at a second, lower end of the grommet. It should beunderstood that relative directional terms are used herein for referenceonly in connection with the drawings and the grommet and othercomponents may be utilized in any position, including those other thanthat shown. Grommet 40 has a configuration which conforms to the shapeof the oval openings 36 in header portion 38 which are sized to receivetube ends 23 from the header core (FIG. 1).

The assembly of the tube ends of the header core 20 into the headeropenings 36 utilizing the grommets is depicted in FIGS. 5, 6 and 7. InFIG. 5, both the tube 22 and grommet 40 are initially aligned along thedirection of the length of the tube. As shown in FIG. 6, the grommet 40is inserted into opening 36 of the header, and the tube end 23 isbrought into contact with the grommet. Then, as shown in FIG. 7, tubeend 23 is forced through the resilient inner opening of grommet 40 sothat the tube 22 is fully received by the grommet within header opening36. Referring back to FIG. 4, the grommets are preferably molded ingroups of more than one, for example a group of three (3), so that theymay be inserted three at a time into the header openings. Once all thegrommets are inserted into the header openings, all of the tube ends 23from the core assembly 20 are inserted at the same time through thegrommet openings.

Preferably the inner diameter of the inward lip of the grommet is lessthan the outer diameter of the tube end, so that after full insertion ofthe tube, the outer wall, the lower end of grommet 40 is forced radiallyoutward. Because of the resilience of the grommet material the tubestretches the grommet in the lower region and flexes its walls toprovide good sealing contact between the inward lip and the outsidesurface of the tube end 23 to form a tight seal around the tube.Additionally, the expansion of the inward lip bulges or forces outwardgrommet outer wall portion which locks against and forms a seal with theedge of the header portion opening to prevent the grommet from slippingback out of the header portion opening. Sealing of the grommet to thetube and the header portion is further enhanced| by the action ofinternal pressure inside the tank/header assembly 60 which serves toforce the lip radially inward into tighter contact with both the tubeand header portion in the manner of an o-ring seal. The inside surfaceof the upper end of the grommet may also be forced inward to contact thetube outer surface to seal against leakage. Additionally, the loweroutside edge of outward lip may also seal against the edge of the headerportion opening to provide an additional seal.

In FIG. 8 there is shown the preferred box header 30 attached andassembled to the core block 20 by means of grommets 40 which hold thetube ends 23 within the header openings 36 to create a sealed,tube-to-header joint. A header is shown both at the upper and lowerportions of the core block attached to both the upper and lower tubeends.

In order to provide against collapse of the oval tubes during insertioninto the grommets in the header openings, an internal reinforcing memberor stiffener may be utilized across the opening of one or more of thetube ends as depicted in FIGS. 9, 10 and, 11. Tube stiffener 50comprises a short flat length of metal or alloy comparable to the tubematerial which may be brazed or welded in place. Unlike internal finswhich may be utilized in some oval tubes, tube stiffener 50 extends onlyfrom approximately tube end 23 to the portion of the tube surrounded bythe grommet, and does not extend substantially further into the body oftube 22 within the fin array of the core assembly. Although onestiffener 50 is depicted in approximately the central portion of thetube opening, additional stiffeners may be employed along the opening oftube end 23. Such stiffeners 50 are particularly useful for tubes havinga minor diameter of 0.5 inch (1.3 cm) to 1 inch (2.5 cm) or more. Insuch cases the internal stiffeners would extend in about 1 inch (2.5 cm)from each end of each tube to provide internal support againstcollapsing under the compressive sealing forces of the grommet. Bylimiting the extent of stiffeners to only the end areas of the tubes,internal flow restriction of the tubes is minimized.

Assembly of the tank to the header 30 receiving the tube end through thegrommets is depicted in FIG. 12. A tank portion 60 having an open endformed by side walls 62 and end walls 64 is aligned with header 30. Thelower edges 62 b of the tank side walls and lower edges 64 b of the tankend walls are aligned with header side wail edges 32 b and header endwall edges 34 b, respectively. After contacting the respective edges ofthe tank 60 and header 30, the seams formed thereby may be welded by MIGor TIG welding procedures, or other well-known welding methods, to formone or more sealed, fluid-tight seams or joints. Both MIG and TIGwelding procedures are preferred because they minimize heat flow intothe header 30.

The completed heat exchanger is depicted in FIG. 13 in which a tank 60is welded both to the upper and lower header portions. The upper tank 60has inlet 66 for receiving the liquid coolant, which then travelsthrough the tank and tubes in core section 20 to lower tank 60 and outthrough outlet 68. Preferably a structural side piece of welded ormechanically fastened bracing is fastened to the inlet and outlet tankand walls.

Thus, the heat exchanger and method depicted herein, utilizing resilienttube header joints, has greatly extended service life compared to commonheat exchangers because of the reduction or elimination of tube toheader stresses. The present invention facilitates assembly of thegrommeted header to the core block because both sides of the grommet areaccessible during assembly. This permits use of bullet noses on thetubes if necessary (to facilitate insertion of the tube ends through thegrommets), backing up of the remote side of the grommet to resist tubeforces from the tube insertion site and inspection of the finished tubeto header joints from both sides of the header. The present inventionalso permits leak testing of the core assembly and adjustment of thetube to header joint before attachment of the tanks. The presentinvention preferably makes use of multiple grommets connected togetherto reduce the possibility of pushing out of the grommets during tubeinsertion. Additionally, the internal stiffeners prevent collapse of thetube ends due to grommet compression sealing forces. The presentinvention permits the use of drawn, cast or fabricated and welded tanks,all to be welded to the box headers. This provides manufacturingflexibility required to provide models, prototypes and short runproduction using fabricated and welded tanks, while using drawn or casttanks for low cost, high volume production.

While the present invention has been particularly described inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. A combinedcore and header assembly for a heat exchanger comprising: a coreassembly having a plurality of tubes connected by a fin array, saidtubes having ends and openings in the ends, the tube ends extending fromthe fin array on at least one side of said core assembly; resilient,polymeric grommets disposed around the tube ends; and a header portionhaving openings receiving said tube ends and said grommets to create asealed, fluid-tight tube-to-header joint, said header portion comprisinga base having said openings therein and side walls on all sides of thebase extending out of a plane of said base and connectable by one ormore fluid-tight welds to a tank portion, said header portion being freeof a tank such that the tube-to-header joint is accessible on both sidesof said joint for inspection and testing before attachment to a tankportion, all of said side walls having free edges extending a distanceout of the plane of said base sufficient to dissipate heat generated inwelding said free edges to a tank portion from adversely affecting thesealing ability of said grommets when said welds are made between saidtank portion and said header portion while said tube ends and grommetsare received in the header portion openings.
 2. The assembly of claim 1wherein said header portion comprises a unitary sheet of metal havingfolded side walls extending from said base.
 3. The assembly of claim 1wherein said header portion comprises a unitary sheet of metal havingfolded side walls and end walls extending from said base, said sidewalls and end walls being joined at corners thereof to create an openbox structure.
 4. The assembly of claim 3 wherein the header portionbase is substantially planar and said side and end walls aresubstantially perpendicular to a plane of said base.
 5. The assembly ofclaim 1 wherein said header portion comprises a unitary sheet of metalhaving drawn side walls extending from said base.
 6. The assembly ofclaim 5 wherein the header portion base is substantially planar and saidside and end walls are substantially perpendicular to a plane of saidbase.
 7. The assembly of claim 1 wherein said grommets comprise a heatresistant silicone rubber sufficient to avoid affecting the sealingability of the grommet when the header portion is welded to the tankportion while the grommets are in the header portion openings.
 8. Theassembly of claim 1 wherein at least a portion of said tube endsincludes a reinforcing member across the openings of said ends, saidreinforcing member extending from the openings of the tube ends to theportion of said tube ends around which the grommet is disposed withoutsubstantially extending into the fin array portion of said coreassembly.
 9. A combined core and header assembly for a heat exchangercomprising: a core assembly having a plurality of tubes connected by afin array, said tubes having ends and openings in the ends, with thetube ends extending from the fin array on at least one side of said coreassembly; resilient, polymeric grommets disposed around the tube ends;and a header portion having openings receiving said tube ends and saidgrommets to create a sealed, fluid-tight tube-to-header joint, saidheader portion comprising a substantially planar base having saidopenings therein and side and end walls extending out of a plane of saidbase and connectable by one or more fluid-tight welds to a tank portion,said header portion being free of a tank such that the tube-to-headerjoint is accessible on both sides of said joint for inspection andtesting before attachment to a tank portion, all of said side and endwalls having free edges extending at least one-half inch out of theplane of said base to dissipate heat generated in welding said freeedges to a tank portion from adversely affecting the sealing ability ofsaid grommets when said welds are made between said tank and headerportion while said tube ends and grommets are received in the headerportion openings.
 10. The assembly of claim 9 wherein said headerportion comprises a unitary sheet of metal having drawn or folded sidewalls and end walls extending substantially perpendicular to the planeof said base, said side walls and end walls being joined at cornersthereof to create an open box structure.
 11. The assembly of claim 10wherein said grommets comprise a heat resistant silicone rubbersufficient to avoid affecting sealing ability of the grommet when theheader portion is welded to the tank portion while the grommets are inthe header portion openings.
 12. The assembly of claim 9 wherein atleast a portion of said tube ends include a reinforcing member acrossopenings of said ends, said reinforcing member extending from theopenings of the tube ends to the portion of said tube ends around whichthe grommet is disposed without substantially extending into said finarray of said core assembly.